The sedimentology of a Late Pleistocene drumlin near Kingscourt, Ireland

Abstract

An exposure in a Late Pleistocene drumlin near Kingscourt, Ireland, provides a good insight into some of the processes that give rise to such subglacial bedforms. The drumlin is located on a ridge, cored by red sandstone of Carboniferous (Namurian) age, which rises to 150 m above m.s.l. The drumlin itself is about 380 m long by 170 m wide and is 20 m in height. It is orientated VVNW-ESE. Ice flow direction in the area, as inferred from general drumlin orientation, striae, and erratic dispersal, was NW-SE. The drumlin is composed of diamicton (with four constituent facies), containing large-scale (up to 20 m long and 1.5 m high) slabs of the sandstone bedrock, which have been displaced tens of metres by ice dragging. The thin diamicton matrix is sand-rich with few clasts greater than pebble size. It contains green sandstone erratics from west and northwest of the study site along with clasts of weathered Namurian sandstone which have been sheared from local bedrock. The diamicton attains a maximum thickness of 5.4 m. The slabs are confined to the basal 2.5 m of the diamicton. Thus the drumlin is essentially rock cored. Parts of the matrix: are interpreted as injection sediments that have been squeezed into fractures and voids in the bedrock, and between rock slabs, under high porewater pressures, when the ice began to displace fractured substrate. Deformation structures, at a variety of scales are seen in the matrix. Deformation, erosion and deposition were all important in the formation of the drumlin. Much of the upper part of the diamicton has been sheared, subsequent to initial deposition, by ice action. The sheared units are uppermost in the stratigraphic sequence. The shearing is the last glacial process apparent in the drumlin sediment and may have been contemporaneous with drumlinisation. Both the squeezing process, which deposited the injections within the matrix, and the deflection in flow of ice were influenced by the obstructing bedrock ridge which, in this case, is also responsible for the anomalous orientation of the feature.